I'd say Zannah acquitted herself pretty well offensively in the final novel. I'd really like to read that and Shadow Killer again to get a better comparison of her speed and agility vs Maul. Though she focused more on defense and Maul on all out attacking, she did well against Bane at his peak, and as evidenced by the rain feat his speed was ridiculous to stand against by that point.
I'd say Kun, but he's specifically a master of HIS weapon, which wasn't a traditional saberstaff. So he may or may not have the same level of skill with a standard saberstaff as he did with his own custom weapon. He was definitely far more skilled with his weapon than Darth Maul or Darth Zannah ever were though.
It's interesting that someone in the thread mention Hord for possible inclusion on the list. Not discounting that at all, but I find it funny that nothing was spoken against that, but Kas'im who came after him and was said to possibly have been the best duelist up to that point--having lived after Hord--is laughed off by so many on these forums.
Well, yes, because he was stated to be one of the greatest warriors in the empire even prior to the invasion.
"Even before the Great War, Malgus was widely regarded as one of the greatest warriors in the Sith Empire." - Swtore, pg 168.
And I mean, Darach was the Jedi Battlemaster. So Malgus beating him is pretty good for his cred.